1. Field of the Invention
This invention relates to zinc-containing copper alloys (typically referred to as brass). More particularly, the resistance of brass to elevated temperature stress relaxation is increased by a controlled addition of alloying elements.
2. Description of Related Art
Throughout this patent application, all compositions are in weight percent, unless otherwise specified.
Alpha brasses, are single phase alloys of copper and zinc that contain up to 39% of zinc. The alloys are characterized by good formability, moderate strength, modest electrical conductivity and low cost. Their combination of strength, formability and electrical conductivity suit the alpha brasses for manufacture into electrical connectors used in appliance and automotive applications.
A limitation on the use of alpha brasses in certain connector applications is inadequate resistance to stress relaxation when the connector operating temperature is significantly above room temperature (nominally, room temperature is 20xc2x0 C.). The connector operating temperature is affected by both the ambient operating temperature and resistance heating (I2R) from the electrical current carried through the connector.
In one method of manufacturing an electrical connector, a wrought sheet of copper alloy is formed into a cantilever spring contact contained within a hollow box. Electrical continuity of a circuit between the connector""s spring contact and a removable blade is assured when a contact force between the spring contact and the inserted blade is maintained at above a design minimum force. Under these conditions, the connection is electrically transparent.
Over time, and more rapidly at elevated temperatures, stress relaxation weakens the contact force between the cantilever spring contact and the blade and may eventually lead to connector failure through an unacceptably low contact force. It is a primary objective of electrical connector design to maximize the contact force between the cantilever spring contact and the blade to maintain a good electrical conductivity path through the connection.
The loss of more than 30% of the originally imposed stress (70% stress remaining) at the product design life (typically 3,000 hours for automotive connectors) is one commonly applied criterion for alloy selection.
Alpha brasses such as copper alloy C240 (nominal composition 78.5%-81.5% copper, balance zinc) and copper alloy C260 (nominal composition 68.5%-71.5% copper, balance zinc) satisfy the 30% loss of originally imposed stress criterion at temperatures only up to about 75xc2x0 C., well below the 125xc2x0 C.-150xc2x0 C. highest anticipated service application temperature for a number of under-the-hood automotive applications.
The addition of other alloying elements to an alpha brass have, typically, not led to an increase in stress relaxation resistance without a significant detrimental effect on other alloy properties, such as conductivity or formability. For example, copper alloy C688 (nominal composition 22.7% zinc, 3.4% aluminum, 0.4% cobalt and remainder copper) has a 75xc2x0 C. application capability, the same as copper alloy C240. While copper alloy C240 has an electrical conductivity of 32%, copper alloy C688 has an electrical conductivity of only 18% IACS. IACS stands for International Annealed Copper Standard and assigns xe2x80x9cpurexe2x80x9d copper an electrical conductivity value of 100% IACS at 20xc2x0 C.
The addition of tin to copper alloy C220 (nominal composition 89%-91% copper, balance zinc) forms copper alloy C425 (nominal composition 9.5% zinc, 1.8% tin, balance copper). Copper alloy C425 has improved stress relaxation resistance enabling the alloy to be formed into connectors having an application temperature of 125xc2x0 C. This advantage is offset by a large decrease in electrical conductivity, from 44% IACS for copper alloy C220 to 28% IACS for copper alloy C425.
U.S. Pat. No. 4,362,579 entitled xe2x80x9cHigh-Strength-Conductivity Copper Alloyxe2x80x9d by Tsuji is incorporated by reference in its entirety herein. The patent recites a copper alloy that is disclosed as having a combination of high strength, excellent electrical conductivity, corrosion resistance and spring qualities. The copper base alloy contains 0.4-8% nickel, 0.1-3% silicon, 10-35% zinc, concomitant impurities and the remainder is copper. The electrical conductivity of the disclosed alloys is relatively low, ranging from 19.1% IACS to 21.2% IACS. Additionally, the required addition of silicon typically decreases hot workability, electrical conductivity and formability.
U.S. Pat. No. 5,820,701 entitled xe2x80x9cCopper Alloy and Process for Obtaining Samexe2x80x9d by Bharghava discloses, in one embodiment, a copper alloy that consists essentially of 1.0%-4.0% tin, 9.0%-15.0% zinc, 0.01%-0.2% phosphorous, 0.01%-0.8% iron, 0.001%-0.5% nickel and/or cobalt and the balance essentially copper. The disclosed copper alloys contain a minimum of 1% of tin.
There remains, therefore, a need for an alpha brass base alloy having an electrical conductivity in excess of 25% IACS and sufficient resistance to stress relaxation that a connector formed from the alloy has a 3,000 hour operating life in the 125xc2x0 C.-150xc2x0 C. temperature regime.
Accordingly, it is an object of the invention to provide an alpha brass base alloy with improved resistance to stress relaxation and an electrical conductivity in excess of 20% IACS. It is feature of the invention that controlled amounts of nickel, tin and phosphorus are added to the base alloy. Another feature of the invention is that the alloys of the invention are capable of forming a uniform and fully recrystallized microstructure. This microstructure is characterized by a very fine grain structure with a uniform dispersion of fine phosphide particles.
Among the advantages of the alloys of the invention are that the alloys have good resistance to stress relaxation at temperatures of up to 125xc2x0 C., and in certain embodiments, the resistance to stress relaxation is significant at temperatures of up to 150xc2x0 C. Another advantage of the alloys of the invention is that the electrical conductivity is not significantly reduced below that of a non-modified alpha brass. Further, the alloys have good bend formability and relatively high yield strength. The alloys of the invention are particularly suitable for forming electrical connectors that are exposed to elevated temperature, such as connectors for automotive applications.
In accordance with the invention, there is provided a modified brass alloy that consists essentially of from 2% to the maximum of zinc that maintains an alpha brass microstructure, from 0.2% to 2% of nickel, from 0.15% to 1% of tin, from 0.03% to 0.35% of phosphorus and the balance is copper and inevitable impurities.
The objects, features and advantages recited above will become more apparent from the specification and drawings that follow.